Japanese Unexamined Patent Application Publication No. 2001-85748 (hereinafter referred to as patent document 1) and Japanese Unexamined Patent Application Publication No. 2001-148514 (hereinafter referred to as patent document 2) propose a light emitting device which comprises an LED chip, a circuit board mounting the LED chip, a metal frame (e.g. made of aluminum) surrounding the LED chip on a surface of the circuit board, and an encapsulation member (e.g. made of a transparent resin such as an epoxy resin and a silicone resin) filled within the frame to encapsulate the LED chip and bonding wires connected to the LED chip. The frame disclosed in the Patent document 1 and 2 is shaped to have an opening area which becomes greater as it is spaced further away from the circuit board and is finished to have a mirror interior face which serves as a reflector reflecting a light emitted from the LED chip.
Further, the patent document 2 discloses the light emitting device configured to give a white luminescent spectrum by use of a blue LED chip with blue luminescence and a yellowish fluorescent material, which is dispersed in a transparent resin encapsulating the blue LED chip to realize the white color light emission by the light from the blue LED chip.)
When an epoxy resin is used as a material of the encapsulation member in the light emitting device, bonding wires may be cut off when subjected to a heat cycle test (temperature cycle test) of repeating a low temperature period of −40° C. and a high temperature period of 80° C. alternately, due to thermal-expansion of an electrically conductive pattern mounted on a substrate of a circuit board in the high temperature environment. In addition, when used as a material of the encapsulation member, the epoxy resin exhibits only lower weatherability than the silicone one.
On the other hand, when a silicone resin is used as a material of the encapsulation member in the light emitting device, the bonding wires can be prevented from being cut off in the high temperature period of the heat cycle test because of that the encapsulation member is gelatinous and elastic. However, since a linear expansion coefficient of the silicone resin, which is a material of the encapsulation member, is more than 10 times that of an aluminum which is a material of the frame, voids may be generated within the encapsulation member due to a difference in linear expansion coefficient between silicone resin and aluminum.
Furthermore, in the light emitting device, although the light emitted from the LED chip can be taken to external of the encapsulation member effectively by means of the frame with a mirror interior face, there is a failure of causing light loss resulting from the reflection of the light on the interior face of the frame.
Besides, in the light emitting device according to the Patent document 1, when a lens controlling an orientation of the light emitted from the LED chip is arranged over the encapsulation member and the frame, a light output may be lowered due to a deviation in an optical axis between the LED chip and the lens, resulting from dimensional inaccuracy of the flame and the lens or positioning inaccuracy thereof.
The Patent document 2 also discloses the light emitting device in which the encapsulating member of the LED chip and the bonding wires connected thereto are configured to have its portion shaped into a convex lens. However, the encapsulation member is likely to suffer from an external force to develop a stress transmittable to the LED chip and the bonding wires. The transmitted stress possibly acts to alter a luminescent characteristic of the LED chip and break the bonding wires, or bring transference of external moisture to the LED chip.